Ceramics materials are now of great interest because of heat resistance, abrasion resistance, high-temperature strength and other useful properties. However, it is very difficult to machine ceramics because they are hard and brittle. For this reason, ceramic articles are generally prepared by a powder sintering method comprising molding powder ceramic material into a desired shape as by compaction and sintering the molded material. Also useful is a precursor method comprising melting an organic polymer as a ceramic precursor or dissolving the polymer in a solvent and shaping the melt or solution into a desired shape. The shaped polymer is then sintered to convert the material into inorganic. The precursor method has the advantage that there can be prepared ceramic articles of complex shapes which would otherwise be almost impossible to form by the powder sintering method. Articles of special shape such as fibers and sheets can be prepared by the precursor method.
Among materials generally known as ceramics, SiC and Si.sub.3 N.sub.4 are particularly of great importance because of their high-temperature properties. More particularly, SiC has good heat resistance and high-temperature strength and Si.sub.3 N.sub.4 is excellent in thermal shock resistance and fracture toughness. The inventors proposed a process for manufacturing organic silazane polymers for use in the manufacture of SiC-Si.sub.3 N.sub.4 ceramic materials by the precursor method and a process for manufacturing ceramics from the polymers as disclosed in Takamizawa et al., U.S. Pat. No. 4,771,118, U.S. Serial No. 114,111, filed Oct. 27, 1987, now U.S. Pat. No. 4,869,854 or West German Offenlegungsschrift DE 3736914 A1, and Japanese Patent Application No. 313264/1987.
In general, ceramic materials are prepared from ceramic precursors by melting, molding, and then infusibilizing the ceramic precursors. The infusibilized precursors are then pyrolyzed into ceramic materials. The infusibilizing step involved in this procedure is usually carried out by several well-known methods. There were proposed various methods including (1) air oxidation, (2) exposure to steam or steam and oxygen, (3) ultraviolet exposure, (4) electron beam exposure, and (5) use of various organic silicon compounds.
These methods have the following problems. Methods (1) and (2) which require only heating in air are widely used because of ease of treatment. However, these methods not only need a great amount of thermal energy, but also yield ceramic materials having a high content of oxygen at the sacrifice of high strength, high modulus, and other characteristics inherent to ceramics.
Unlike methods (1) and (2), methods (3) and (4) have advantages that the energy cost is reduced and they avoid contamination with oxygen. However, these methods require an increased dose of ultraviolet or electron radiation for practically sufficient infusibilization, and equipment for such exposure is very expensive. These methods are commercially unacceptable in these respects.
Method (5) is typically by infusibilizing polymers having an R.sub.3 SiNH- radical with various organic silicon compounds such as silicon tetrachloride and trichlorosilane or metal chlorides such as BCl.sub.3 and SnCl.sub.4 as disclosed in U.S. Pat. No. 4,535,007. The inventors have found that this method is not effective at all t organic silazane polymers free of an R3SiNH- radical. When processed by this method, fibers of organic silazane polymers free of an R.sub.3 SiNH- radical are fusion bonded together, losing their own shape as will be demonstrated later in Comparative Example. Since this U.S. Patent refers nowhere to the strength of ceramic fibers after pyrolysis which is the most important factor in the ceramic precursor method, it is unknown how the method is effective in infusibilizing.
Therefore, there is a need for eliminating the abovementioned drawbacks of the prior art ceramic precursor infusibilizing methods.